Republican Curtis Bostic, fighting a David-and-Goliath battle against former South Carolina Gov. Mark Sanford in a GOP congressional primary runoff, landed his first major national endorsement today from Rick Santorum.

“I am pleased to announce our endorsement of Curtis Bostic in South Carolina’s first congressional district,” said Santorum, who waged his own against-the-odds campaign against Mitt Romney for the Republican presidential nomination in 2012. “Curtis’ groundswell finish to a runoff spot signals a win for the strength of his grassroots supporters and his robust conservative values.”

Santorum, who co-founded the 501(c)4 advocacy group Patriot Voices with his wife, Karen, is expected to campaign for Bostic in the 1st District later this week. “We must continue that momentum for Curtis into the runoff election, and I look forward to joining conservatives across South Carolina in helping to elect Curtis Bostic to the U.S. House of Representatives,” Santorum said.

The former Pennsylvania senator’s endorsement comes with just a week remaining until the April 2 runoff that will pit Bostic, a former Charleston County council member, against Sanford, who got 37 percent of the vote in a 16-candidate field in the March 19 Republican primary. The winner of next Tuesday’s runoff will face Democrat Elizabeth Colbert Busch, sister of cable-TV comedian Stephen Colbert, in the May 7 special election to fill the 1st District House seat vacated by Tim Scott’s appointment to the Senate.

In a conference call with bloggers last week, Bostic described himself as a “constitutional conservative” and said his campaign has “strong evangelical support.” The endorsement by Santorum, a devout pro-life Catholic, can be expected to energize Bostic social conservative supporters.

UPDATE: An e-mail announcement of the Bostic endorsement sent to the Patriot Voices list states, “Senator Santorum will travel to South Carolina this Wednesday to support Curtis Bostic. Details of that schedule will be released shortly.”

UPDATE III: Jennifer Rubin of the Washington Post says the SC-1 race “is a test of of Santorum’s sway with the GOP’s social conservative wing. If he wants to remain viable for 2016 (or outside of presidential politics), there could be no bigger boost than helping to bring down a character like Sanford.”